1. Field of the Invention
The present invention relates to semiconductor fuses and, more particularly, to a semiconductor fuse that provides an open current path when programmed without exposing the fuse to the environment.
2. Description of the Related Art
A semiconductor fuse is a device that provides a low-resistance current path between two conductive lines when the fuse is unprogrammed, and a highresistance current path between the two conductive lines when the fuse is programmed.
FIG. 1A shows a plan view that illustrates a conventional semiconductor fuse 100. FIG. 1B shows a cross-sectional diagram taken along line 1B--1B of FIG. 1A. As shown in FIGS. 1A and 1B, fuse 100, which is formed on a first layer of isolation material 110, includes a strip of doped polysilicon (poly) 112 which is formed on isolation layer 110.
As further shown in FIGS. 1A and 1B, fuse 100 also includes a second layer of isolation material 114 which is formed on poly strip 112. Isolation layer 114, in turn, has a number of openings at each end of poly strip 112 that expose a corresponding number of regions on the surface of poly strip 112.
In addition, fuse 100 further includes a number of contacts 116 which are formed in the openings in isolation layer 114 to be electrically connected to poly strip 112. Fuse 100 also includes a first metal line ML1 which is formed on isolation layer 114 to be electrically connected to the contacts 116 at one end of poly strip 112, and a second metal line ML2 which is formed on isolation layer 114 to be electrically connected to the contacts at the other end of poly strip 112.
Fuse 100 further includes a third layer of isolation material 120 (not shown in FIG. 1A) which is formed on isolation layer 114 and metal lines ML1 and ML2. In addition, a window opening 122 is formed in isolation layers 114 and 120 to expose the top surface of poly strip 112. (One common variation is to form a thin layer of oxide 124 on the exposed top surface of poly strip 112 as shown by the dashed line in FIG. 1B.)
In operation, in the native or unprogrammed state, which is shown in FIGS. 1A and 1B, fuse 100 provides a low-resistance current path between metal lines ML1 and ML2. When programmed, however, a portion of fuse 100 is removed which, in turn, provides an open current path between metal lines ML1 and ML2.
Fuse 100 is programmed by applying a voltage drop across metal lines ML1 and ML2 which causes a programming current to flow from metal line ML1 through poly strip 112 to metal line ML2. The programming current must have a magnitude which is sufficient to heat poly strip 112 to approximately 1400.degree. C.
At this temperature, the portion of poly strip 112 exposed by window opening 122 boils away, escaping through window opening 122. The process is self limiting in that when enough polysilicon has boiled away to open the current path, the programming current ceases to flow through poly strip 112, thereby removing the source of heat.
FIG. 2 shows a cross-sectional diagram that illustrates fuse 100 after fuse 100 has been programmed. As shown in FIG. 2, poly strip 112 is broken into two electrically disconnected strips 112A and 112B as a result of the programming current passing through poly strip 112.
(When oxide layer 124 is formed on the exposed top surface of poly strip 112, the heating causes oxide layer 124 to crack which, in turn, provides a path for the polysilicon to escape.)
One of the disadvantages of fuse 100 is that poly strip 112 (or oxide layer 124) and the adjacent circuitry is exposed to the outside environment via window opening 122. As a result of this exposure, fuse 100 and the adjacent circuitry is subject to contamination from the outside environment.
To eliminate this source of contamination, other approaches for forming fuses have also been developed which do not rely on the removal of a portion of poly strip 112. One of these other approaches uses a strip of material which, when heated, changes from a low-resistivity state to a high-resistivity state.
FIG. 3A shows a cross-sectional diagram that illustrates a conventional unprogrammed fuse 300 which changes resistivity states when heated. FIG. 3B shows a cross-sectional diagram that illustrates fuse 300 after fuse 300 has been programmed. Fuse 300 is similar to fuse 100 and, as a result, utilizes the same reference numerals to designate the structures which are common to both fuses. As shown in FIGS. 3A-3B, fuse 300 differs from fuse 100 in that fuse 300 has no window opening formed in isolation layer 120, but instead has a layer of silicide 310 formed on the top surface of poly strip 112. When unprogrammed, silicide layer 310 reduces the resistivity of poly strip 112.
On the other hand, when poly strip 112 and silicide layer 310 are heated to a mixing temperature via a programming current, the chemical reaction between the polysilicon and the silicide forms an agglomeration 312 which is significantly more resistant to current flow than the silicided poly strip.
One of the disadvantages of fuse 300 is that, although fuse 300 eliminates window opening 122, fuse 300 provides a high-resistance current path rather than an open current path. Although the current flowing through fuse 300 when fuse 300 is programmed is very small, there is always a desire to further minimize power consumption for battery-powered devices. Another disadvantage of fuse 300 is that the inclusion of silicide layer 310 increases the fabrication cost of fuse 300.
As a result, there is a need for a fuse which eliminates window opening 122 while at the same time providing an open current path after programming.